1. Field of the Invention
The invention relates to a process for discharging impurities from a suspension containing paper fibers using a hydrocyclone in which diluting liquid is added into the suspension in the radially inner area of the hydrocyclone. The invention also relates to a hydrocyclone for carrying out the process.
2. Discussion of Background Information
As is known, hydrocyclones are well suited to concentrating the heavy parts and/or the light parts of a pulp suspension. Such devices typically utilize centrifugal forces in order to conduct such parts out of the hydrocyclone through a discharge aperture provided for this purpose. As a rule, hydrocyclones also serve to remove impurities such as small metal parts, glass slivers, sand, styropor, as well as light plastics. Moreover, it is important for such devices to achieve a high separating effect with increased operational reliability, e.g., freedom from clogging. In these known devices, clogs can occur when the impurities collect together and become concentrated in specific locations, e.g., in the area of the discharge aperture provided for them. These concentrations can accumulate over time in such a way that they result in a blockage. In many cases, a compromise must be found between the operational reliability of the device, e.g., the avoidance of clogs, and the highest possible efficiency of the machine, e.g., throughput and/or separating effect. One known method which attempts to address the problem of clogging utilizes the feeding in a diluting liquid in heavy-part hydrocyclones. Such a device utilizes a water feed line which opens in an edge area of the hydrocyclone, e.g., through slits in the cyclone wall in the peripheral direction. The diluting liquid acts to cause eddies at the edge layers. However, this design suffers from the fact that the diluted liquid must penetrate the heavy soil that is already concentrated therein, which can lead to a back washing of heavy parts into the accepted stock stream.
Other solutions utilize a central pipe having apertures facing radially outwards for adding the diluting liquid. For example, Canadian Patent Application No. 1 138 378 A describes a device wherein the diluting liquid quickly reaches the wall of the hydrocyclone with the detrimental effects already described. In such a design, undesired rebound effects can arise owing to the short distance and the flow of the diluting liquid directly onto the cyclone wall. Moreover, there is also an increased risk of clogging of the apertures, which are kept relatively small. In addition, the suggested swirl flow leads to further interfering eddies.
A known design is described in U.S. Pat. No. 3,785,489. This device utilizes an axial outflow. However, the design also produces a rotating flow at the central inflow of the diluting water by way of a number of vanes. Accordingly, this flow quickly drives the diluting water outwards. The effect of this flow is also an increased risk of clogging.